This invention relates to a media weight sensor of the type that includes a transducer consisting of a metal disk with a piezoelectric element fabricated on one side which form the back of a Helmholtz resonator cavity mounted in a printer so that the media going to the printer moves across the top of the Helmholtz resonator where an opening of the resonator is located. A soft, polymeric roller may be used to press the media against the transducer. The resonant frequency of the Helmholtz resonator is affected by the media. The mass of the media adds to the mass of the resonator, thereby lowering the resonant frequency. Consequently, the heavier the media, the more the resonant frequency is lowered.
It is known, in paperweight sensors, to employ optical sensors. Exemplary of such prior art is U.S. Pat. No. 5,138,178 (""178) to L. F. Wong et. al., entitled xe2x80x9cPhotoelectric Paper Basis Weight Sensorxe2x80x9d and U.S. Pat. No. 5,127,643 (""643) to A. T. DeSanctis et. al., entitled xe2x80x9cAutomatic Copy Sheet Selection Device.xe2x80x9d While the ""178 and ""643 references employ optical sensors, these sensors are used to measure thickness or weight of the paper. These measurements are accomplished by measuring the amount of light that passes through the paper. However, if the paper is coated, this coating can adversely affect how much light passes through the paper. Consequently, an accurate measurement may not be obtained.
It is also known, in paperweight sensors, to measure the stiffness of the paper in order to determine the weight of the paper. Exemplary of such prior art is commonly assigned U.S. Pat. No. 5,962,861 (""861) to P. Fowler, entitled xe2x80x9cSheet Media Weight Detector and Methodxe2x80x9d and commonly assigned U.S. Pat. No. 6,028,318 (""318) to W. L. Cornelius, entitled xe2x80x9cPrint Media Weight Detection System.xe2x80x9d While the ""861 and ""318 references measure the stiffness of the paper in order to ascertain the weight of the paper, these do not employ an acoustic resonator. Instead, these references measure the deflection of the paper that is related to the stiffness and, thereby the weight of the paper.
Finally, it is known, in paperweight sensors, to measure paper thickness. Exemplary of such prior art is U.S. Pat. No. 5,806,992 (""992) to Y. Ju, entitled xe2x80x9cSheet Thickness Sensing Technique and Recording Head Automatic Adjusting Technique of Ink Jet Recording Apparatus Using Same.xe2x80x9d While the ""992 reference measures sheet thickness, it does so by measuring the amount of arm rotation, which can result in a complex and fragile assembly. While the apparatus of the ""992 reference may be able to accurately measure the thickness of the sheet of paper, in order to determine the weight of the paper, assumptions must be made as to the makeup of the sheet of paper. For example, it must be assumed that each sheet of paper has the same density. However, it is well known that the density of sheets of paper in the same stack of paper can vary by as much as a factor of two. Consequently, a weight determination cannot be accurately made.
It is apparent from the above that there exists a need in the art for a media weight sensor system which is lightweight through simplicity of parts and uniqueness of structure, and which at least equals the media weight sensing characteristics of the known media weight sensors, but which at the same time employs an acoustic resonator. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
Generally speaking, this invention fulfills these needs by providing a media weight sensing apparatus, comprising a Helmholtz resonator means having an opening substantially located in one end of the resonator means and a media weight measuring means operatively connected to the other end of the resonator means and a media traversing means for traversing a media, whose weight is to be determined, across the opening in the resonator means.
In certain preferred embodiments, the Helmholtz resonator includes a housing, a piezoelectric element, and a metal disk. Also, the media weight measuring means includes a drive circuit operatively connected to the piezoelectric element. Finally, the media traversing means includes a compliant roller.
In another further preferred embodiment, the apparatus measures a media property that is a combination of both the media thickness and density. As a result, the measurement may more accurately reflect the media weight by measuring the change of the resonant frequency of the piezoelectric element with and without the media. Since it is a differencing measurement, it will be relatively insensitive to factors, such as wear and temperature.
The preferred sensing apparatus, according to this invention, offers the following advantages: lightness in weight; ease of assembly and repair; excellent weight measurement characteristics; good stability; excellent durability; and good economy. In fact, in many of the preferred embodiments, these factors of lightness in weight, ease of assembly and repair, weight measurement characteristics, and durability are optimized to an extent that is considerably higher than heretofore achieved in prior, known media weight sensing apparatus.
The above and other features of the present invention, which will become more apparent as a description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: